As is well known, the brightness of a subject illuminated by a flash unit on a camera varies inversely as the square of the distance of the subject from the flash. If the aperture area of a shutter is selected such that it is related to the distance of the subject to the flash by the inverse of the relationship between brightness and distance (i.e., directly as the square of the distance), then under conditions of negligible ambient light, a predetermined amount of light will pass through the exposure aperture independently of subject distance. By properly selecting the parameters of flash intensity and aperture area, the predetermined amount of light passing through the exposure aperture will properly expose a sheet of film located behind the shutter.
If subject distance is measured in a manner that is directly related to time (e.g., by using an ultrasonic range finder), then the brightness of a subject due to flash illumination will be a parametric function of time (i.e., will vary inversely as the square of time). Furthermore, if a scanning shutter is utilized such that the exposure aperture established by the shutter varies directly as the square of time measured from first light, then the product, at any instant, of the aperture area and the brightness of the subject due to its flash illumination will be a constant. By providing apparatus that fires the flash subsequent to first light after a period of time proportional to subject distance, the exposure aperture at the instant of firing of the flash will have an area such that the amount of light reflected from the subject due to its illumination by the flash and passing through the aperture will properly expose the film, provided only that negligible ambient light is present.
Apparatus described above is disclosed in U.S. Pat. No. 4,188,103. Such apparatus relies on a scanning shutter whose aperture size follows what is termed a time-squared program. Upon actuation of this shutter, the exposure aperture increases as the square of time measured from first light, and the output of an ultrasonic rangefinder fires the flash after a lapse of a period of time subsequent to first light which is directly proportional to the distance of the subject determined by the rangefinder. Under conditions of flash illumination (i.e., no significant ambient light) the amount of light which is reflected from the subject due to its illumination by the flash and which passes through the exposure aperture will be independent of subject range enabling a proper exposure to be made each time the shutter is actuated regardless of subject distance up to the maximum distance at which the flash is effective.
This technique is effective as long as the shutter follows a time-squared program. If the program of a given shutter varies from actuation to actuation, then erratic exposures will result. In addition, the configuration of the apertures of the overlying displaceable blades of the scanning shutter is critical. Under some conditions, it is not possible to design the apertures of a scanning shutter with a time-squared program such that the shape of the exposure aperture is optimum.
It is therefore an object of the present invention to provide a new and improved method of and apparatus for actuating a flash unit on a camera utilizing a scanning shutter to achieve optimum exposure wherein the results are independent of the program of the scanning shutter thus permitting optimal shaping of the aperture area throughout the program.